In earth formations traversed by a borehole and wherein there are a plurality of petroleum bearing strata at different borehole depths, it is common practice to insert a plurality of tubing strings into the uncased borehole and then, by cementing the borehole, to isolate the strata outside the tubing strings from each other so that there may be provided a plurality of petroleum production zones. Multiple completion wells may in some cases involve many producing zones; with two to four producing zones being guite common. Tubing strings are usually run so that within a given production zone there may be a plurality of tubing strings. After the tubing strings are installed and cemented, it is necessary to perforate the tubing string that is to carry the output of a particular production zone. This perforating is accomplished by lowering a perforating device (usually a shaped charge gun) within the tubing string to be perforated, then, orienting the device so that only the desired tubing string will be perforated, and then actuating the perforating device. It is essential in every case that the perforating device shall be fired so as to avoid perforation of any other tubing string.
Various types of orienting devices have been utilized in the prior art. Among these are devices which employ an electromagnetic field as the phenomena indicative of an azimuthal pattern, as disclosed, for example, by U.S. Pat. No. 3,704,749. The electromagnetic field is generated by use of an excitor coil, and the alternating current power for the excitor coil is usually transmitted from above ground to the downhole tool via the single conductor cable or wire line from which the downhole tool is suspended. The presence of alternating current on the wire line presents certain safety problems, since the perforator gun is usually detonated by a negative signal that is transmitted to the downhole tool via the wire line. To resolve such safety problems to an acceptable degree requires measures which increase the complexity of the downhole tool.
It is an object of this invention to provide a borehole orienting tool which does not require the transmission of alternating current power via the wire line to the downhole tool.
Another object of the invention is to provide improved borehole orienting apparatus and systems.
These and other objects are effected by the invention as will be apparent from the following description taken in accordance with the accompanying drawings, forming a part of this application.